Denaturization is a process that proteins undergo when they are subjected to
stresses that will affect the structure of the molecule. So first, let's talk
about protein structure.
Proteins are very long molecules containing 100 or more amino acids linked
together. This long chain of amino acids is the primary structure of the
protein but a long, straight chain is not stable; it requires a lot of energy
to maintain it. So, proteins will form a coil, like a spring, that requires
less energy to maintain. This coil is called the alpha helix and is the
secondary structure of the protein. Once the protein is coiled, different
groups that are attached to the amino acids in the chain become close to each
other and may form bonds to further stabilize the molecure; this is called the
tertiary structure. This final structure is very important when discussing the
function of the protein in the body and in food preparation. Most proteins have
either a globular (like a ball) or fibrous (long, kind of stringy) tertiary
structure.
So now, back to denaturization. When a protein is stressed, like during heating
(cooking an egg), stirring (whipping up egg whites) or exposure to UV light,
the bonds that hold a protein in its tertiary structure will begin to break.
When these bonds break, the protein starts to unfold and loses some its
properties. For example, denatured proteins usually becomes less soluble, that
is, it doen't dissolve in water as well. If the protein is an enzyme, it will
lose its ability to function as an enzyme. If the stress that is causing the
denaturation continues, other changes may occur. Now that the normal structure
of the protein is gone, new bonds may be formed, giving it a different shape.
Proteins next to each other may form bonds between them so now you have a big
clump of proteins all hooked together. At this stage, its called coagulation.
This is what happens when you cook an egg and why the color changes from clear
(lots of space between protein molecules for light to show through) to white
(not very much space for light to go through). Sometimes you want proteins to
denature, like when you cook an egg and sometimes you don't want that to
happen, like cooking milk so much that it curdles. The curdled parts are the
proteins that have been denatured and coagulated into a big clump. This isn't
very nice if you are making cream soup and you want it smooth and creamy, not
lumpy.
A perfect exercise to do to demonstrate denaturization and coagulation is to
whip egg whites. When you first start out, they are foamy but if you stop they
will eventually sink back and look that same as when you started. But, as you
continue to beat them, the proteins get more and more denatured, so they start
to hold up even when you aren't beating them and become white. And if you keep
doing it, they will begin to form clumps and the whole thing collapses. At this
point, the egg white is coagulated and nothing you do can "fix" it - that is
make it uncoagulated. These two processes are physical changes, not chemical
changes to the structure of the protein. The same amino acids are still there
as when you started; just in a different shape.
Now, all living things have protein in them but some more than others, so that
for something like vegetables, what happens when they cook has more to do with
the starches that make up the cell walls. Vegetables don't lose protein when
you cook them, although some denaturation does occur, just like I described
above. But the cell wall structure is so rigid in vegetables that you really
have to cook them a long time (longer than even the most overcooked vegetable
would be) before you end up with coagulated proteins.
This is a very long answer but I hope I explained it ok. The study of proteins
and what happens to them when you cook or in the body is fascinating.
Jill Irvin, RD
resources:
Foods, Experimental Perspectives; Margaret McWilliams
Introductory Foods, Marion Bennion